Among the many pathways controlling cell proliferation and differentiation, genes of the retinoblastoma protein (Rb) regulatory network stand out as frequent if not obligatory targets for mutation or deregulation during tumorigenesis. Although biochemical and tissue culture studies have implicated Rb family members in a wide range of cellular activities, the bona fide functions of Rb in vivo and during normal development are not well understood. Our long-term objective is to understand the cellular, and developmental functions of Rb family proteins at the molecular level. To this end, we have devised a genetic strategy that has allowed us to identify genes and pathways that function coordinately with the C. elegans Rb homolog, lin-35, to control essential developmental processes. Using this system, we have demonstrated canonical cell cycle functions for LIN-35 as well as a novel role for this protein in organ morphogenesis. We have also uncovered a complementary pathway that acts to control organ morphogenesis through UBC-18/UbcH7, a conserved ubiquitin-conjugating enzyme involved in the targeting of proteins for degradation. The proposed experiments are designed to uncover the underlying mechanism by which LIN-35, acting in conjunction with one or more parallel pathways, regulates organ morphogenesis in C. elegans. Our main objectives fall into two categories. One broad aim is to identify additional factors that function cooperatively with LIN-35 and UBC-18 to control organogenesis. These studies will include the cloning and characterization of sir-9, a gene that, like ubc-18, functions redundantly with lin-35 to control organ morphogenesis;the execution of a two-hybrid screen to identify UBC-18-interacting proteins;and a directed RNAi feeding screen using known or putative ubiquitin pathway components. Our second objective is to identify functionally relevant downstream targets for regulation by LIN-35 and UBC-18. These studies will include genetic selections to isolate mutations that suppress the lethality of lin-35;ubc-18 double mutants;microarray analyses to identify the complete spectrum of LIN-35-regulated transcripts;and additional two-hybrid screens using co-factors of UBC-18 identified through earlier two-hybrid or RNAi-feeding screens. The successful completion of these studies will greatly enhance our general understanding of Rb family functions and will provide detailed mechanistic knowledge of this novel role for Rb proteins in morphogenesis.